Hearing assistance devices, such as hearing aids, include, but are not limited to, devices for use in the ear, in the ear canal, completely in the canal, and behind the ear. Such devices have been developed to ameliorate the effects of hearing losses in individuals. Hearing deficiencies can range from deafness to hearing losses where the individual has impairment responding to different frequencies of sound or to being able to differentiate sounds occurring simultaneously. The hearing assistance device in its most elementary form usually provides for auditory correction through the amplification and filtering of sound provided in the environment with the intent that the individual hears better than without the amplification.
Hearing aids employ different forms of amplification to achieve improved hearing. However, with improved amplification comes a need for noise reduction techniques to improve the listener's ability to hear amplified sounds of interest as opposed to noise.
Many methods for multi-microphone noise reduction have been proposed. Two methods (Peissig and Kollmeier, 1994, 1997, and Lindemann, 1995, 1997) propose binaural noise reduction by applying a time-varying gain in left and right channels (i.e., in hearing aids on opposite sides of the head) to suppress jammer-dominated periods and let target-dominated periods be presented unattenuated. These systems work by comparing the signals at left and right sides, then attenuating left and right outputs when the signals are not similar (i.e., when the signals are dominated by a source not in the target direction), and passing them through unattenuated when the signals are similar (i.e., when the signals are dominated by a source in the target direction). To perform these methods as taught, however, requires a high bit-rate interchange between left and right hearing aids to carry out the signal comparison, which is not practical with current systems. Thus, a method for performing the comparison using a lower bit-rate interchange is needed.
Roy and Vetterli (2008) teach encoding power values in frequency bands and transmitting them rather than the microphone signal samples or their frequency band representations. One of their approaches suggests doing so at a low bitrate through the use of a modulo function. This method may not be robust, however, due to violations of the assumptions leading to use of the modulo function. In addition, they teach this toward the goal of reproducing the signal from one side of the head in the instrument on the other side, rather than doing noise reduction with the transmitted information.
Srinivasan (2008) teaches low-bandwidth binaural beamforming through limiting the frequency range from which signals are transmitted. We teach differently from this in two ways: we teach encoding information (Srinivasan teaches no encoding of the information before transmitting); and, we teach transmitting information over the whole frequency range.
Therefore, an improved system for improved intelligibility without a degradation in natural sound quality in hearing assistance devices is needed.